/Engine/Plugins/FX/Niagara/Source/NiagaraEditor/Public/ViewModels/HierarchyEditor/NiagaraSummaryViewViewModel.h
Can be overridden to support custom sections. In the hierarchy only categories can be parented directly to the root, but using this it is possible to add items to custom sections in the source panel. This will only work for top-level objects, i.e. anything directly under the root.
virtual const [UNiagaraHierarchySection](API\Plugins\NiagaraEditor\ViewModels\HierarchyEditor\UNiagaraHierarchySection) * GetSectionInternal() const